title: Cell density regulates in vitro activation of heart valve interstitial cells.
abstract: BACKGROUND
  Valve interstitial cells, the most prominent cell type in the heart valve, are activated and express α-smooth muscle actin in valve repair and in diseased valves. We hypothesize that cell density, time in culture, and the establishment of cell-cell contacts may be involved in regulating valve interstitial cell activation in vitro.
  
  
  METHODS
  To study cell density, valve interstitial cells were plated at passages 3 to 5, at a density of 17,000 cells/22 × 22 mm(2) coverslip, and grown for 1, 2, 4, 7, and 10 days. Valve interstitial cells were stained for α-smooth muscle actin and viewed under confocal microscopy to characterize the intensity of staining. To study time in culture, valve interstitial cells were plated at a 10-fold higher density to achieve similar growth densities over a shorter time period compared with valve interstitial cells plated at low density. α-Smooth muscle actin staining was compared at the same time points between those plated at high and low densities. To confirm valve interstitial cell activation as indicated by α-smooth muscle actin staining, valve interstitial cells were stained for cofilin at days 2, 5, 8, and 14 days postplating. To study the association of transforming growth factor β with valve interstitial cell activation with respect to cell density, valve interstitial cells were stained for α-smooth muscle actin and transforming growth factor β at 2, 4, 6, and 8 days postplating. To study the activation of the transforming growth factor β signaling pathway, valve interstitial cells were stained for pSmad2/3 at days 2, 4, 6, 8, 10, and 12 days postplating. To study cell contacts and activation, subconfluent and confluent cultures of valve interstitial cells were stained for β-catenin, N-cadherin, and α-smooth muscle actin. Also, whole-cell lysates of subconfluent and confluent valve interstitial cell cultures were probed by Western blot analysis for phospho-β-catenin at Ser33/37/Thr41, which is the form of β-catenin targeted for proteosomal degradation.
  
  
  RESULTS
  The percentage valve interstitial cells with high-intensity α-smooth muscle actin staining decreases significantly between days 1 and 4, and at confluency, most cells show absent or low-intensity staining, regardless of time in culture. Similar results are obtained with cofilin staining. Transforming growth factor β and nuclear pSmad2/3 staining in valve interstitial cells decreases concurrently with valve interstitial cell activation as cell density increases. Examining β-catenin and N-cadherin staining, single valve interstitial cells show no cell-cell contact with strong cytoplasmic staining, with some showing nuclear staining of β-catenin, while confluent monolayers show strong staining of fully established cell-cell contacts, weak cytoplasmic staining, and absent nuclear staining. The presence of cell-cell contacts is associated with a decreased α-smooth muscle actin. The level of phospho-β-catenin at Ser33/37/Thr41 is lower in confluent cultures compared with low-density subconfluent valve interstitial cell cultures.
  
  
  CONCLUSIONS
  Cell-cell contacts may inhibit valve interstitial cell activation, while absence of cell-cell contacts may contribute to activation.
authors.affiliation: Department of Pathology, Toronto General Research Institute, University Health Network, Ontario, Canada.
authors.forename: Songyi
authors.initials: S
authors.lastname: Xu
authors.forename: Amber Chang
authors.initials: AC
authors.lastname: Liu
authors.forename: Hyunjun
authors.initials: H
authors.lastname: Kim
authors.forename: Avrum I
authors.initials: AI
authors.lastname: Gotlieb
mesh_terms: D000199:Actins / Q000378:metabolism; D000818:Animals; D002450:Cell Communication / Q000502:physiology*; D002452:Cell Count; D049109:Cell Proliferation; D002478:Cells, Cultured; D008943:Mitral Valve / Q000166:cytology* / Q000378:metabolism; D015398:Signal Transduction; D022502:Stress Fibers / Q000378:metabolism*; D013552:Swine; D016212:Transforming Growth Factor beta / Q000378:metabolism; D014945:Wound Healing / Q000502:physiology*; D051176:beta Catenin / Q000378:metabolism
chemical_list: D000199:Actins; D016212:Transforming Growth Factor beta; D051176:beta Catenin